Automatic Antenna Tuning Units (ATUs) are well known in the field of High Frequency (HF) (2-30 MHz) radio transceiver design, where radios and antennas must be designed to operate over wide frequency bands. High frequency ATUs are typically designed to handle high power levels, such as one kilowatt, and may utilize, for example, motor-driven variable capacitors and inductors that are capable of withstanding many kilovolts of RF (Radio Frequency) voltage and many amps of current. Such ATUs are, however, bulky, expensive, and operationally slow.
A typical HF ATU operates by initiating a tuning phase after a change to the transmitter operating frequency. During the tuning phase, a carrier signal, often unmodulated, is transmitted while the ATU adjusts the variable circuit elements and searches for the minimum VSWR (Voltage Standing Wave Ratio) condition. (As is well known in the art, VSWR is one way to express impedance mismatch, which causes signal reflections in a circuit.) The status of the ATU is then frozen until the next frequency change or until manual re-initiation of the tuning phase.
In other frequency bands, such as the Very High Frequency (VHF) band (30-100 MHz), the antenna Q (quality)-factors are generally much less than those found in HF antennas. As a result, VHF ATUs may be made using step-tuned inductors or capacitors, with associated relays or PIN diodes for switching in or out the correct combination of components. Typically, a set of switch commands are pre-determined for each frequency channel and stored in a read-only memory. These switch commands select the correct combination of matching components. Thus, upon a change in frequency, the stored switch commands can be retrieved from the memory for the new frequency channel and used to operate the various switches.
This type of antenna matching permits frequency hopping radios to be made where the antenna is tuned for each new hop frequency. The antenna tuning typically occurs in a time period between each new frequency hop during which no transmission takes place. This time period is typically known as the guard time. However, in prior art frequency hopping, an impedance mismatch observed on a previous frequency hop is generally uncorrelated with the mismatch seen on a later frequency hop. The stored tuning commands for the subsequent frequency hop are simply retrieved from memory to switch in or out the correct combination of matching components. In other words, there is no adaptive correction based on previous mismatches.
In current applications, wireless communication devices, such as cellular phones, must operate at various frequencies in the RF, HF, VHF, UHF (Ultra-High Frequency) or low microwave bands to transmit and receive signals in, for example, a Time Division Multiple Access (TDMA) network. Absent a tuning device, the antenna impedance observed by the transceiver circuits is a function of the operating frequency, and may also vary substantially depending upon the proximity of the antenna to the human body. Therefore, it may be insufficient to determine fixed matching commands for the various channel frequencies, such as in conventional frequency hopping radios, due to the varying proximity of the cellular phone, and hence the antenna, to a user's body. Furthermore, the proximity of the cellular phone to the user's body may vary during a call, necessitating the detection and correction of a resultant impedance change without interrupting the call or otherwise distorting the signal. There is therefore a need for very small, low-cost, adaptive antenna matching techniques that are capable of operating continuously during normal transceiver use.
In the above-referenced patents, methods are disclosed for using a transceiver's receiver section during active signal transmissions to evaluate the complex antenna reflection coefficient. These methods further include adapting antenna matching components in response to this evaluation. These previously described methods may be conveniently implemented in transceivers utilizing a homodyne receiver, where the phase- or frequency-modulated transmit signal may be used as the local oscillator for the homodyne receiver. In such an arrangement, the phase or frequency modulation on the reflected transmitter signal cancels with the same modulation on the receiver local oscillator. Thus, measurements of the antenna reflection are automatically compensated for the transmitter phase modulation.
More recently, linear modulation, comprising both phase and amplitude modulation, has entered common use as a way of achieving improved communications capacity, system flexibility, and/or higher data rates. In transceivers utilizing such phase-amplitude modulation schemes, antenna reflection measurements using the previously disclosed methods do not yield results that are automatically compensated for the transmitter modulation. Accordingly, there is a need for an improved method of measuring antenna reflection, compensated for advanced modulation schemes, as well as for providing periods during which the receiver is not employed for receiving user data which may be re-employed for such compensated measurements.